Resistivity Size Effect Research Articles

Characterization method of conductive film state of micro-cathode arc thrusters

Micro-Cathode Arc Thruster (μCAT) is an advanced electric propulsion system developed in the past decades to meet the maneuvering requirements of space microsatellites. However, the constraint on the its application is the short lifespan of the thruster. This is attributed to the conductive film, a key component of thruster discharge. In this study, we propose a measurement method for μCAT conductive film, and enables the evaluation of the evolution process of film thickness. The method is based on the four-probe resistivity testing theory and size effect fitting model, and establishes a method for characterizing the thickness and resistivity of μCAT conductive films. A comparison was made between the corrected resistivity test results of 1, 2, 5, and 10 μm film and the fitting results of the titanium film, revealing an error ofapproximately 10%. Additionally, a total of 2500 ignition experiments were conducted to verify the film thickness and resistance measurement of this method. This research serves as a valuable basis for the lifetime investigation of μCAT. Consequently, the thickness of the film progressively decreases with each discharge cycle, and the resistivity exhibits a corresponding increase due to the size effect. This method could be used as an effective method for exploring the μCAT lifespan and performance.

Anomalous size effect of impact resistance in carbon nanotube film

Dynamic mechanical behavior and size-related impact resistance of CNT films are studied by employing laser-induced projectile impact test (LIPIT) and coarse-grained molecular dynamics (CGMD) simulation. The energy dissipation mechanisms of the CNT films are investigated via CGMD simulations. An evident anomalous thickness-dependent effect is directly observed in the experiment, consistent with simulation phenomena. The mechanisms underlying this anomalous thickness-dependent effect are investigated at the atomic scale. The disparities between experiments and simulations are discussed. Our analysis of energy dissipation modes, deformation behaviors during impact, and impact area reveals that kinetic energy change predominantly governs the deformation mode. Meanwhile, a plugging failure mode near the exit face of CNT film is identified at high impact velocity (∼160 m/s), leading to a deterioration in impact resistance and a corresponding reduction in SEA with increasing CNT film thickness. These findings provide a feasible strategy for the protection design of CNT film in broaden protective application scenarios.

Low-resistivity molybdenum-carbide thin films formed by thermal atomic layer deposition with pressure-assisted decomposition reaction

The rapid increase in the resistivity of thin metal films as their thickness decreases to sub-10 nm, known as the resistivity size effect, is an important issue that must be addressed to ensure device performance in ultraminiaturized semiconductor devices. Molybdenum carbide (MoCx) has been studied as a candidate for emerging interconnection materials because it can maintain a low resistivity even at a low thickness. However, reports on stable precursors with guaranteed reactivity for atomic layer deposition (ALD) remain limited; moreover, the process of forming low-resistance MoCx thin films must be studied. In this study, we propose a new route to form low-resistivity MoCx thin films by thermal ALD with partial ligand dissociation by controlling the process pressure to enhance the reactivity of the Mo precursor with reactants. Following the proposed deposition process and subsequent annealing, uniform and continuous thin films were formed (even at a sub-5 nm thickness), with an extremely low resistivity of approximately 130 μΩ cm. Therefore, the proposed method can be applied as a next-generation interconnect process; notably, high-quality thin films can be formed through pressure-assisted decomposition, even with a lack of thermal energy during the ALD process.

Resistivity size effect in epitaxial face-centered cubic Co(001) layers

Metastable face-centered cubic (fcc) Co layers are deposited by reactive magnetron sputtering in 5 mTorr N2 at 400 °C followed by vacuum annealing at 500 °C. The resulting phase-pure Co(001)/MgO(001) layers contain negligible nitrogen and exhibit a surface roughness <0.8 nm and a cube-on-cube epitaxial relationship with the substrate with Co[100]ǁMgO[100]. The measured resistivity vs thickness d = 10–1000 nm indicates a bulk resistivity ρo = 6.4 ± 0.3 μΩ cm for fcc Co at room temperature and ρo = 1.3 ± 0.1 μΩ cm at 77 K, and an effective electron phonon mean free path λ = 27 ± 2 nm and 79 ± 6 nm at 295 and 77 K, respectively. The resulting ρo × λ benchmark quantity is 3–5 times larger than that predicted from first principles, suggesting a breakdown of the Fuchs–Sondheimer model at small dimensions. The overall results indicate that fcc Co exhibits no intrinsic conductance benefit over stable hcp Co nor conventional Cu for narrow interconnects. The developed method for growth of epitaxial fcc Co(001) layers provides opportunities to study this metastable material for potential spintronic applications.

Electrical Resistivity Modification of Electrodeposited Mo and Mo–Co Nanowires for Interconnect Applications

Achieving historically anticipated improvement in the performance of integrated circuits is challenging, due to the increasing cost and complexity of the required technologies with each new generation. To overcome this limitation, the exploration and development of novel interconnect materials and processes are highly desirable in the microelectronics field. Molybdenum (Mo) is attracting attention as an advanced interconnect material due to its small resistivity size effect and high cohesive energy; however, effective processing methods for such materials have not been widely investigated. Here, we investigate the electrochemical behavior of ions in the confined nanopores that affect the electrical properties and microstructures of nanoscale Mo and Mo–Co alloys prepared via template-assisted electrodeposition. Additives in an electrolyte allow the deposition of extremely pure metal materials, due to their interaction with metal ions and nanopores. In this study, boric acid and tetrabutylammonium bisulfate (TBA) were added to an acetate bath to inhibit the hydrogen evolution reaction. TBA accelerated the reduction of Mo at the surface by inducing surface conduction on the nanopores. Metallic Mo nanowires with a 130 nm diameter synthesized through high-aspect-ratio nanopore engineering exhibited a resistivity of (63.0 ± 17.9) μΩ·cm. We also evaluated the resistivities of Mo–Co alloy nanowires at various compositions toward replacing irreducible conventional barrier/liner layers. An intermetallic compound formed at a Mo composition of 28.6 at%, the resistivity of the Mo–Co nanowire was (58.0 ± 10.6) μΩ·cm, indicating its superior electrical and adhesive properties in comparison with those of conventional barriers such as TaN and TiN. Furthermore, density functional theory and non-equilibrium Green’s function calculations confirmed that the vertical resistance of the via structure constructed from Mo-based materials was 21% lower than that of a conventional Cu/Ta/TaN structure.

Resistivity size effect in epitaxial VNi2 layers

Epitaxial VNi2 layers are deposited onto MgO(001) and their resistivity ρ measured as a function of layer thickness d = 10.5–138 nm to quantify the resistivity size effect. The layers exhibit resistivity minima at both stoichiometric V:Ni = 1:2 composition and 700 °C growth temperature, which is attributed to electron scattering at V–Ni antisite defects and is described using the ordering parameter within the framework by Rossiter. A cube-on-cube epitaxy of the fcc parent structure on MgO(001) leads to two possible layer orientations for orthorhombic VNi2(010) and VNi2(103), resulting in considerable atomic disorder at domain boundaries, consistent with relatively small x-ray coherence lengths of 8 and 14 nm in-plane and along the growth direction of a 33.5 nm thick layer. In situ ρ vs d measurements yield a bulk resistivity of ρo = 46 ± 2 μ Ω cm and a benchmark quantity of ρoλ = (138 ± 5) × 10−16 Ω m2, where λ is the bulk electron mean free path. Air exposure causes a minor resistivity increase due to 2 ± 1 nm thick surface oxide that perturbs the surface potential. Resistivities at 77 K are Δρ = 16 ± 3 μ Ω cm below those at room temperature. This Δρ is thickness independent and is close to the previously predicted 13.9 μ Ω cm bulk resistivity for VNi2 along [100]. However, the measured bulk resistivity is well above this prediction, which is attributed to electron scattering at domain boundaries/atomic disorder. Consequently, the theoretically predicted superior directional conduction cannot be experimentally confirmed in this study. The overall results indicate that VNi2 is only a promising compound for narrow interconnects if a synthesis scheme can be developed that results in a strong atomic order, a negligible domain boundary density, and a [100] crystalline orientation along the transport direction.

Size effect of resistivity due to surface roughness scattering in alternative interconnect metals: Cu, Co, Ru, and Mo

The resistivity size effect of thin films due to atomically rough surfaces is investigated using first-principles quantum transport simulations with the disorder scattering treated by the nonequilibrium mean-field approach. Within the exact muffin-tin orbital--based first-principles method, the Madelung potential of film in device structure is constructed by implementing the boundary-condition correction. Cu(001), Co(0001), Ru(0001), and Mo(001) thin films are modeled with the thickness $d=1--10$ nm. The random surface roughness is represented by an alloy model, consisting of one monolayer of ${\text{M}}_{x}{\text{Va}}_{1\ensuremath{-}x}$ and ${\text{M}}_{1\ensuremath{-}x}{\text{Va}}_{x}$ on the respective top and bottom surfaces. The results of all metal films indicate that the first-principles resistivity ${\ensuremath{\rho}}_{s}$ induced by surface roughness scattering is proportional to $1/d$. Our simulated resistivity results are consistent with the experimental measurements of epitaxial metal layers. We find that, for the same thickness, Mo films present the highest ${\ensuremath{\rho}}_{s}$, significantly larger than the other metals. The thin-film resisitivity ${\ensuremath{\rho}}_{s}$ of Co is about 1.6 times that of Cu, while Ru results is slightly higher than Cu results. For all metal films, we obtain the parameter ${\ensuremath{\gamma}}_{s}$ characterizing the intensity of surface roughness scattering as a function of $x$. Furthermore, we find the proportionality constant ${\ensuremath{\alpha}}_{s}$ versus $x$ for the mean-free path ${\ensuremath{\lambda}}_{s}={\ensuremath{\alpha}}_{s}\ifmmode\times\else\texttimes\fi{}d$ for surface roughness scattering. Our results show that at the high $x>0.2, {\ensuremath{\alpha}}_{s}$ is rather close to a constant, with values of 4.5, 2.8, 2.1, and 1.0 for the respective Cu, Ru, Co, and Mo. We conclude that, compared to Co, Ru is competitive in resistivity size effect of surface roughness for an alternative to Cu interconnect for future technology nodes.

Anisotropic Resistivity Size Effect in Epitaxial Mo(001) and Mo(011) Layers

Mo(001) and Mo(011) layers with thickness d = 4-400 nm are sputter-deposited onto MgO(001) and α-Al2O3(112¯0) substrates and their resistivity is measured in situ and ex situ at room temperature and 77 K in order to quantify the resistivity size effect. Both Mo(001) and Mo(011) layers are epitaxial single crystals and exhibit a resistivity increase with decreasing d due to electron surface scattering that is well described by the classical Fuchs and Sondheimer model. Data fitting yields room temperature effective electron mean free paths λ*= 14.4 ± 0.3 and 11.7 ± 0.3 nm, respectively, indicating an anisotropy with a smaller resistivity size effect for the Mo(011) orientation. This is attributed to a smaller average Fermi velocity component perpendicular to (011) surfaces, causing less surface scattering and a suppressed resistivity size effect. First-principles electronic structure calculations in combination with Boltzmann transport simulations predict an orientation dependent transport with a more pronounced resistivity increase for Mo(001) than Mo(011). This is in agreement with the measurements, confirming the effect of the Fermi surface shape on the thin-film resistivity. The predicted anisotropy λ001*/λ011* = 1.57 is in reasonable agreement with 1.66 and 1.23 measured at 77 and 295 K. The overall results indicate that the resistivity size effect in Mo is relatively small, with a measured product of the bulk resistivity times the effective electron mean free path ρoλ* = (7.7 ± 0.3) and (6.2 ± 0.2) × 10-16 Ωm2 for Mo(001) and Mo(011) layers. The latter value is in excellent agreement with the first-principles-predicted ρoλ = 5.99 × 10-16 Ωm2 and is 10% and 40% smaller than the reported measured ρoλ for Cu and W, respectively, indicating the promise of Mo as an alternate conductor for narrow interconnects.

Resistivity scaling in CuTi determined from transport measurements and first-principles simulations

The resistivity size effect in the ordered intermetallic CuTi compound is quantified using in situ and ex situ thin film resistivity ρ measurements at 295 and 77 K, and density functional theory Fermi surface and electron–phonon scattering calculations. Epitaxial CuTi(001) layers with thickness d = 5.8–149 nm are deposited on MgO(001) at 350 °C and exhibit ρ vs d data that are well described by the classical Fuchs and Sondheimer model, indicating a room-temperature effective electron mean free path λ = 12.5 ± 0.6 nm, a bulk resistivity ρo = 19.5 ± 0.3 μΩ cm, and a temperature-independent product ρoλ = 24.7 × 10−16 Ω m2. First-principles calculations indicate a strongly anisotropic Fermi surface with electron velocities ranging from 0.7 × 105 to 6.6 × 105 m/s, electron–phonon scattering lengths of 0.8–8.5 nm (with an average of 4.6 nm), and a resulting ρo = 20.6 ± 0.2 μΩ cm in the (001) plane, in excellent agreement (7% deviation) with the measurements. However, the measured ρoλ is almost 2.4 times larger than predicted, indicating a break-down of the classical transport models. Air exposure causes a 6%–30% resistivity increase, suggesting a transition from partially specular (p = 0.5) to completely diffuse surface scattering due to surface oxidation as detected by x-ray photoelectron spectroscopy. Polycrystalline CuTi layers deposited on SiO2/Si substrates exhibit a 001 texture, a grain width that increases with d, and a 74%–163% larger resistivity than the epitaxial layers due to electron scattering at grain boundaries. The overall results suggest that CuTi is a promising candidate for highly scaled interconnects in integrated circuits only if it facilitates liner-free metallization.

Size-Dependent Grain-Boundary Scattering in Topological Semimetals

We assess the viability of topological semimetals for application in advanced interconnect technology, where conductor size is on the order of a few nanometers and grain boundaries are expected to be prevalent. We investigate the electron transport properties and grain-boundary scattering in thin films of the topological semimetals $\mathrm{Co}\mathrm{Si}$ and $\mathrm{Co}\mathrm{Ge}$ using first-principles calculations combined with the nonequilibrium Green's function (NEGF) technique. Unlike conventional interconnect metals such as $\mathrm{Cu}$ and $\mathrm{Al}$, we find that $\mathrm{Co}\mathrm{Si}$ and $\mathrm{Co}\mathrm{Ge}$ conduct primarily through topologically protected surface states in thin-film structures even in the presence of grain boundaries. The area-normalized resistance decreases with decreasing film thickness for $\mathrm{Co}\mathrm{Si}$ and $\mathrm{Co}\mathrm{Ge}$ thin films both with and without grain boundaries; a trend opposite to that of the conventional metals $\mathrm{Cu}$ and $\mathrm{Al}$. The surface-dominated transport mechanisms in thin films of topological semimetals with grain boundaries does not follow the classic resistivity size effect, and suggests that these materials may be promising candidates for applications as nanointerconnects where high electrical resistivity acts as a major bottleneck limiting semiconductor device performance.

(Digital Presentation) On Epitaxial Electrodeposition of Co, Ru and Cu for Interconnect Applications

As the critical dimensions of copper (Cu) interconnects approach dimensions near the mean free path of the metal (39.9 nm at room temperature), a rise in resistivity is observed. This phenomenon, termed resistivity size-effect, is the result of electron scattering at grain boundaries and surfaces. Given that electron scattering from grain boundaries is a major contributor to the resistivity rise in nanoscale interconnects, in this study we demonstrate epitaxial electrodeposition of single crystal and bicrystal films, using Co, Ru and Cu as three metals of choice. In detail, we will address the choice of seed layer, the role of symmetry of the seed layer, the sign and magnitude of the misfit strain (compressive, none, tensile), the epitaxial orientation relationship of the electrodeposited film and the seed layer, the impact of underpotential deposition on film nucleation, and the growth morphology. The role of electrolyte, ion concentration and pH will also be addressed.

Resistivity size effect due to surface steps on ruthenium thin films computed with a realistic tight-binding model

A realistic tight-binding model is developed and employed to elucidate the resistivity size effect due to steps on Ru thin films. The resistivity of two different film orientations, (0001) and (11¯00), is computed for transport along the [112¯0] direction both for smooth surfaces and for surfaces with monolayer-high steps. In the case of smooth films, the systems are also studied using solutions to the Boltzmann transport equation. Interestingly, the resistivity of (11¯00) surfaces exhibits a significant size effect even in the absence of surface steps. When monolayer-high steps are spaced ∼10 nm apart, the resistivity is shown to increase due to scattering from the steps. However, only a small increase was found which cannot explain the large effect seen in recent experiments with Ru thin films. This highlights the need for further elucidation of the resistivity size effect. Theoretical analysis suggests that films made from materials with a relatively large ballistic conductance per area like Ru should exhibit a reduced resistivity size effect. This result points to Ru as a promising interconnect material. Finally, because a very efficient algorithm for computing resistivity based on the kernel polynomial method is used, the approach fulfills a need for realistic models that can span length scales directly relevant to experimental results. The calculations described here include films approaching 5 nm in thickness, with in-plane distances up to ∼160 nm and 3.8×105 atomic sites.

Resistivity size effect in epitaxial iridium layers

The resistivity size effect in Ir is quantified with in situ and ex situ transport measurements at 295 and 77 K using epitaxial layers with thickness d = 5–140 nm deposited on MgO(001) and Al2O3(0001) substrates. Data fitting with the Fuchs–Sondheimer model of the measured resistivity ρ vs d for single-crystal Ir(001)/MgO(001) layers deposited at Ts = 1000 °C yield an effective electron mean free path λeff = 7.4 ± 1.2 nm at 295 K, a room-temperature bulk resistivity ρo = 5.2 μΩ cm, and a temperature-independent product ρoλeff = (3.8 ± 0.6)×10−16 Ω m2, which is in good agreement with first-principles predictions. Layers deposited at Ts = 700 °C and stepwise annealed to 1000 °C exhibit a unique polycrystalline multi-domain microstructure with smooth renucleated 111-oriented grains that are >10 μm wide for d = 10 nm, resulting in a 26% lower ρoλeff. Ir(111)/Al2O3(0001) layers exhibit two 60°-rotated epitaxial domains with an average lateral grain size of 88 nm. The grain boundaries cause a thickness-independent resistivity contribution Δρgb = 0.86 ± 0.19 and 0.84 ± 0.12 μΩ cm at 295 and 77 K, indicating an electron reflection coefficient R = 0.52 ± 0.02 for this boundary characterized by a 60° rotation about the ⟨111⟩ axis. The overall results indicate that microstructural features including strain fields from misfit dislocations and/or atomic-level roughness strongly affect the resistivity size effect in Ir. The measured ρoλeff for Ir is smaller than for any other elemental metal and 69%, 43%, and 25% below reported ρoλ products for Co, Cu, and Ru, respectively, indicating that Ir is a promising alternate metal for narrow high-conductivity interconnects.

Resistance of single domain walls in half-metallic CrO2 epitaxial nanostructures.

Magnetic domain structures are active electron transport agents and can be used to induce large magnetoresistance (MR), particularly in half-metallic solids. We have studied the excess resistance induced by a single magnetic domain wall in a one-dimensional half-metallic CrO2 nanoscale conductor with a built-in constriction whose channel width (d) ranges from 30 to 200 nm. We observed that the domain wall-induced MR is enhanced by 70 fold when d decreases from 200 nm to 30 nm. We speculate that the enhancement is due to the increased domain wall resistance (DWR) and the extra contribution of ballistic magnetoresistance (BMR). We have uncovered a large size effect of d on the MR induced by the domain wall, which scales with d as d-1.87±0.32. Accordingly, we predict that the MR ratio of a simple CrO2 nanowire impregnated with a constriction at a 150 nm2 cross-section could reach 100%. This large MR far exceeds that of a conventional ferromagnetic nanowire, confirming the role of half metallicity on enhanced magneto-transport.

Resistivity Size Effect in Epitaxial Rh(001) and Rh(111) Layers

Rh(001) and Rh(111) layers with thickness d = 8-181 nm are sputter deposited onto MgO(001) and Al2O3(112̅0) substrates and their resistivity ρ measured in situ and ex situ at room temperature and 77 K in order to quantify the resistivity size effect. Rh(001) layers are epitaxial single crystals and show a resistivity increase with decreasing d that is well described with the classical Fuchs and Sondheimer model, indicating an effective electron mean free path λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> = 9.5 ± 0.8 nm at 295 K. Rh(111) layers exhibit an epitaxial microstructure with two 60°-rotated domains with a lateral width of 19 ± 2 nm, as determined from the X-ray coherence length. The grain boundaries between the domains cause a thickness-independent resistivity contribution Δρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gb</sub> = 0.74 and 0.59 μΩcm at 295 and 77 K, indicating an electron reflection coefficient R = 0.16 ± 0.03 for a boundary characterized by a 60° rotation about the (111) axis. Air exposure causes a nearly negligible (<; 6%) resistivity increase which suggests a possible reduction in the surface scattering specularity by Δp <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> = -0.3 ± 0.2 during surface oxidation. The overall results yield a temperature-independent product of the bulk resistivity times the electron mean free path ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> λ = (4.5 ± 0.4) × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-16</sup> Ωm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This is 1.4 times larger than previously predicted from first principles, but 33%, 55%, 63%, and 11% smaller than for Cu, W, Co, and Ru, respectively, suggesting great promise for Rh as an alternative metal for narrow interconnects.

Size effects of tin oxide quantum dot gas sensors: from partial depletion to volume depletion

The grain size effect is one of the fundamental characteristics of semiconductor gas sensors. However, it has not been fully understood due to the absence of studies on the volume-depleted grains. In this work, the gas-sensitive SnO2 quantum dots (QDs) from partial depletion to volume depletion are prepared to discuss the size effects. A facile aqueous-based method is used to prepare the size-controllable SnO2 QDs of 2.0–12.6 nm. The resistance shows a monotonically negative size effect while the response reaches the optimization when the grain radius is comparable to the depletion layer width. It is suggested that the design of highly sensitive gas sensors should consider the equal importance of the control of grain size and depletion layer width. The computational results illustrate size-dependent energy level of donors and number of quasi-free electrons, which are responsible for the negative size effect of resistivity in the volume-depleted SnO2 crystallites. This work provides a comprehensive understanding of grain size effects from partial depletion to volume depletion in semiconductor gas sensors.

Electron Scattering at Epitaxial Ni(001) Surfaces

Epitaxial Ni(001) layers are sputter deposited on MgO(001) substrates and their electrical resistivity $\rho $ measured in situ as a function of thickness ${d}_{{\text {Ni}}}= $ 5–100 nm to quantify the resistivity size effect due to electron surface scattering. X-ray diffraction $\theta $ – ${2}\theta $ scans, $\omega $ -rocking curves, and pole figures confirm an epitaxial layer-substrate relationship with Ni[001] $\|$ MgO[001] and Ni[100] $\|$ MgO[100]. The resistivity is well described with the semiclassical model by Fuchs and Sondheimer and a room-temperature bulk resistivity $\rho _{o}= {7.04}\,\,\mu \Omega $ cm, yielding a bulk electron mean-free path $\lambda = {26}\pm {2}$ and 350 ± 20 nm at 295 and 77 K, respectively. Air exposure causes a resistivity increase by up to 21%, which is attributed to monolayer surface oxidation that results in a transition from 30% specular to completely diffuse electron surface scattering. All measured data are consistent with a temperature-independent product $\rho _{o} \lambda = {18.3} \times {10}^{{-{16}}}\,\,\Omega \text{m}^{{{2}}}$ , which is 4.5 times larger than previously predicted from first-principles, indicating that Ni is less promising as a metal for narrow interconnect lines than those predictions suggest.

The Resistivity Size Effect in Epitaxial Nb(001) and Nb(011) Layers

Epitaxial Nb(011) and Nb(001) layers are sputter deposited onto ${a}$ -plane and ${r}$ -plane sapphire substrates, respectively, and their resistivity $\rho $ measured in situ , ex situ , and at 77 K as a function of layer thickness ${d}= 4$ –400 nm. The resistivity increase with decreasing ${d}$ is independent of layer orientation and is described with the model by Fuchs and Sondheimer (FS), providing a value for the bulk electron mean free path $\lambda = {20} \pm {2}$ nm at room temperature. Exposure to air causes a 1.5-nm-thick surface oxide and an increase in $\rho $ by up to 74%, suggesting a decrease in the surface scattering specularity from ${p}_{{{1}}}= {0.9} \pm {0.1}$ at the Nb-vacuum interface to completely diffuse scattering ( ${p}_{{{1}}}= {0}$ ) at the oxidized Nb surface. Alternatively, this increase in resistance can be attributed to roughening during surface oxidation while retaining completely diffuse scattering, yielding a lower bound for the room-temperature $\lambda $ of 9.0±0.4 nm. The product of the bulk resistivity $\rho _{o}$ times $\lambda $ is temperature-independent and, depending on either choosing the roughness or the specularity interpretation, $\rho _{o}\,\,\lambda = {14}\times {10}^{{-{16}}}$ or ${30} \times {10}^{{-{16}}}\,\,\Omega \text{m}^{{{2}}}$ , respectively. These values are 3.9 and 8.5 times larger than $\rho _{o}~\lambda $ from a previous theoretical prediction, indicating a dramatic break down of the classical FS model for Nb and indicating that the resistivity size effect in Nb is considerably larger than predicted earlier. They are also larger than for W, Ru, and Co, making Nb not promising for high-conductivity narrow interconnect lines.

Resistivity and surface scattering of (0001) single crystal ruthenium thin films

The resistivity size effect in nanoscale metals is of both scientific and technological interest, the latter due to its importance to interconnects between transistors in integrated circuits. In this work, the authors report the variation of resistivity with film thickness and with changes in surface scattering of ex situ annealed single crystal Ru thin films grown on sapphire substrates by sputter deposition. The room temperature deposition of SiO2 on the Ru sample surface was observed to increase the resistivity of films that had previously been subjected to annealing in a reducing gas ambient. These overcoated samples were also found to increase in resistivity as a result of an oxidizing anneal and reduce in resistivity as a result of a subsequent reducing gas (Ar + H2) anneal. To a large extent, the surface structure and electron scattering characteristics were found to be reversible between oxidizing and reducing gas anneals. The chemistry and structure of the Ru upper surface was characterized by low energy electron diffraction (prior to the SiO2 overcoat deposition), x-ray reflectivity, x-ray photoelectron spectroscopy, and resistivity measurements. The changes in surface structure and chemistry were related to the changes in the specularity of the Ru surface for electron scattering in the context of the Fuchs–Sondheimer semiclassical model of the resistivity size effect, and in this context a mostly specular metal/dielectric interface is reported.

CuAl2 thin films as a low-resistivity interconnect material for advanced semiconductor devices

New interconnect materials that have a low line resistivity are required to address issues associated with the increased resistivity due to the aggressive downscaling of future semiconductor devices. In this work, CuAl2 thin films are investigated as a potential material for liner- and barrier-free interconnect applications. The results show that CuAl2 blanket films adhere well to and do not undergo interdiffusion with SiO2, as well as having a favorable size effect of resistivity. Furthermore, the filling of CuAl2 in narrow low-k trenches is investigated, and an excellent gap-filling performance is registered. These features suggest that CuAl2 is a promising alternative to Cu that does not require any additional liner or barrier layers for feature sizes less than 10 nm.